mac80211: connect with HT20 if HT40 is not permitted

Some changes to fix issues with HT40 APs in Korea
and follow-up changes to allow using HT40 even if
the local regulatory database disallows it caused
issues with iwlwifi (and could cause issues with
other devices); iwlwifi firmware would assert if
you tried to connect to an AP that has an invalid
configuration (e.g. using HT40- on channel 140.)

Fix this, while avoiding the "Korean AP" issue by
disabling HT40 and advertising HT20 to the AP
when connecting.

Cc: stable@vger.kernel.org [3.6]
Reported-by: Florian Reitmeir <florian@reitmeir.org>
Tested-by: Florian Reitmeir <florian@reitmeir.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>